The widely employed system, utilizing a hydrokinetic torque-converter or fluid coupling in conjunction with planetary gearing is known to suffer from the disadvantage of reduced efficiency through slip losses in the torque-converter or fluid coupling, and the present invention has for one of its objects a means of eliminating or reducing this loss when using the higher speed ratios of the transmission.
It is also well known that the characteristics of the internal-combustion engine are such that its specific fuel consumption varies with engine speed, and that fuel economy can therefore be gained by providing such gear ratios between engine and driving wheels that enables the engine to run mainly at the most efficient engine speed that will permit it to provide sufficient power to drive the vehicle at the speed required. For these reasons the present invention has for another of its objects to provide in the transmission unit not only a direct-drive but also a suitable overdrive ratio (one in which the output shaft is driven at a rotational speed greater than that of the input shaft) so that the highest economy can be obtained in the overdrive ratio, while greater acceleration, and in some circumstances greater vehicle speed, is obtainable in the direct-drive ratio. It is a further object of the invention that when either of these two ratios is in use the loss of efficiency inherent in the torque-converter or fluid coupling shall be eliminated or reduced and that the efficiency of the transmission gearing shall be high in these two ratios.
A suitable relationship between the numberical values of the overdrive ratio and the direct drive ratio lies between the limits of 1.25 and 1.4.
According to the invention there is provided a variable speed power transmission comprising a hydrokinetic torque transmitter whose input member is adapted to be driven by a prime-mover and a planetary gear-set disposed coaxially in relation to the torque transmitter and on that side of the latter which is nearer the final output member of the transmission, the said gear-set having a larger sun pinion and a smaller sun pinion, a planet carrier mounted on bearings coaxially with the said sun pinions and with the torque transmitter and provided with selectively operable braking means, a ring-gear secured to an output member of the transmission for rotation therewith coaxially with the planet carrier and having internal teeth disposed in the same transverse plane as that of the smaller sun pinion, a set of short planet pinions and a set of long planet pinions mounted on bearings in the planet carrier, the short planet pinions each meshing with the smaller sun pinion and with the ring-gear, the long planet pinions each meshing with the larger sun pinion and with one or more of the short planet pinions but not engaging either the ring-gear or the smaller sun pinion, characterised in that the smaller sun pinion is disposed on that side of the larger sun pinion which is nearer to the torque transmitter and is provided with selectively operable braking means whereby it may be arrested to provide the reaction for the overdrive ratio and each sun pinion is provided with independent clutching means whereby either or both sun pinions may be drivably connected to the turbine or output member of the torque-converter, and in that the transmission includes further independent clutching means whereby a driving connection may be established between the input member of the torque transmitter and the planet carrier through the medium of a central drive shaft which passes freely through the torque transmitter and through the two sun pinions.
Transmission systems employing torque-converters and planetary gearing have been proposed in which the planetary gearing is disposed coaxially with the torque-converter and has two clutchable input drives, one of which is taken from the converter input member and the other is taken from the converter output member, thus enabling part of the transmitted power to by-pass the converter in the direct-drive ratio. The present invention employs the system so described, but its novelty lies in the arrangement and disposition of the various gear elements so as to provide a suitable overdrive ratio in addition to a direct drive and a reverse drive and reduction ratios, the arrangement providing, in the overdrive ratio, a power path utilizing a minimum number of gear meshes in series in order to achieve high gear efficiency in keeping with the best planetary gear design practice, the arrangement at the same time enabling the direct drive to be taken either wholly or partially through the torque-converter or fluid coupling forming the torque transmitter or to be taken wholly and directly from the engine to eliminate all slip losses in the torque transmitter. The arrangement according to the present invention also provides for at least two reduction ratios (in which the output shaft rotates slower than the input shaft) and a reverse-drive ratio approximately equal to or lower than the lowest reduction ratio, all of which ratios are powered wholly through the torque-converter or fluid coupling.
The planetary gear-set hereinbefore defined can be shown to provide a reverse drive ratio and four forward drive ratios including two reduction ratios, a direct drive and an overdrive ratio. Optionally the said gear-set may be extended in order to provide a supplementary reduction ratio intermediate between the said two reduction ratios, thereby yielding five forward drive ratios including direct drive and overdrive.
The said supplementary reduction ratio is advantageously made available by the introduction of a supplementary ring-gear of smaller pitch-circle than the first mentioned ring-gear and disposed on that side of it which is remote from the output member of the transmission, the supplementary ring-gear being in mesh with the said long planet pinions which, with the planet carrier, may be extended in axial length beyond the plane of the short planet pinions sufficiently to make this possible. The supplementary ring-gear is also provided with independently selectable braking means to hold it against rotation when the supplementary reduction ratio is required.